1. Field of the Invention
This invention concerns a method and apparatus for microbiological analysis of biological samples in liquid suspension by light-scattering technique, as set forth in the respective main claims.
To be more exact, the invention concerns a method and apparatus for monitoring the quantity and quality of colonies of microorganisms present in the samples.
The method according to the invention and the relative apparatus find their proper application in analysis laboratories which perform advantageously, but not only, the microbiological analysis of urine, blood, faeces, etc.
2. Discussion of Prior Art
According to the state of the art the analysis of biological samples such as urine, blood, etc. can be carried out substantially according to two methods depending on the information to be obtained.
In particular, chemical-clinical analysis provides information about the chemical composition of the biological sample and identifies the percentages and concentration of the various chemical elements contained in the sample, whereas microbiological analysis has the purpose of identifying the presence of bacteria, which are important from a pathological point of view, and then of establishing the type and concentration of those bacteria. As regards this latter case various techniques are known at present for the evaluation of the presence/absence, number and species of bacteria contained in those biological samples.
The technique accepted everywhere and most used is based on so-called urinoculture, which consists in the distribution of known volumes of possibly diluted biological samples on plates known as Petri dishes and containing culture media suitable for replication of any bacterial colonies in the samples.
Quantitative evaluation of the bacteria in the culture after a given period of time enables information to be obtained about the initial concentration of these bacteria in relation to the volume of the sample used and the dilution factor employed.
The length of the time required to obtain results in a form which can be evaluated is seldom less than 18 hours and in most cases is about 24 hours.
Moreover, the calculation is based on visual evaluations of the final bacterial distribution and on considerations of a statistical type applied by the operator of the apparatus and therefore does not ensure absolute accuracy of the method.
Furthermore, this method, as it does not have available initial information about the presence/absence of the bacteria, entails a heavy work load and a superabundance of equipment in the event of a great percentage of negative cultures to be evaluated.
Methods are known which make use of the occurrence of special chemical reactions of the biological samples when subjected to given treatments.
These methods give speedy information about the presence/absence of bacteria and thus enable the positive samples to be separated from the negative samples in a short time.
But so as to evaluate the concentration and type of the bacteria in the positive cases thus selected it is always necessary to make use of the method of bacterial culture detailed above.
Moreover, these chemical methods may be upset by aspecific reactions which affect the reliability of the results achieved.
Combined methods of a biochemical type exist which are based on evaluation of the variations of absorbed radiation and colour changes in various media on which the sample being examined is distributed
References to these methods are considered in patents Nos. EP-A-0301699, U.S. Pat. No. A-3,322,956, U.S. Pat. No. A-4,577,970, FR-A-2.350.393.
These latter methods yield qualitative information about the species present in relatively short times of the order of 5 to 12 hours, but the results may not be accurate .
Furthermore, these methods do not provide quantitative type of information and, thus, do not allow to define the curve of replication of microorganisms.
Furthermore, the cost of this evaluation method is especially high, and also the problem remains of having to carry out the whole analysis step also on samples which are then found to be negative.
Next, the traditional methods do not provide information concerning the presence in the sample of corpuscular components such as leukocytes, erithrocytes and salts, and this information has to be obtained by specific analyses of a type different from the types described above.